1. Field of the Invention
The present invention relates to a boost converter which is widely used in a power converting apparatus such as a rectifier, a battery discharging circuit or the like, and more particularly to an improved boost converter using a snubber circuit capable of absorbing a stress energy occurring upon a switching device therein being switched and reproducing it through an LC resonance circuit therein.
2. Description of the Prior Art
A general boost converter, as shown in FIG. 1, comprises a first inductor 12 connected between one terminal (+) of an input portion V.sub.i for receiving an input signal and a first connection point P1, a first diode 14 connected between the first connection point P1 and one terminal (+) of an output portion V.sub.o, and a switching device 13 connected between the first connection point P1 and the other terminal (-) of the input portion V.sub.i, the other terminal (-) of the input portion being connected to the other terminal (-) of the output portion V.sub.o.
In this boost converter, if the switching device 13 is turned on, a current signal from the input portion V.sub.i is applied to the switching device 13 through the first inductor 12 and then fed back to the negative terminal (-) of the input portion V.sub.i. Thus, when the switching device 13 is conductive, a loop is formed which consists of the first inductor 12, the switching device 13 and the input portion V.sub.i. As a result, the magnitude of the current passing through the first inductor 12 is increased.
If, however, the switching device 13 is turned off, the current signal from the input portion V.sub.i is applied to an anode of the first diode 14 through the first inductor 12 and then fed back to the positive terminal (+) of the output portion V.sub.o. Thus, when the switching device 13 is not conductive, a loop is formed which consists of the first inductor 12, the first diode 14, the output portion V.sub.o and the input portion V.sub.i. As a result, the magnitude of the current from the input portion V.sub.i is reduced.
Therefore, in order to control the output of the boost converter, it is necessary to control the ratio of ON/OFF pulse width occurring when the switching device 13 is switched. When switched, the switching device 13 suffers from an excessive stress such as an excessive current stress, or an excessive voltage stress. For this reason, the boost converter needs to have an additional circuit, i.e. a snubber circuit, so as to reduce the excessive stress caused in the switching device 13.
FIG. 2 is a circuit diagram of a prior art boost converter using a snubber circuit. Component elements having similar functions to the component elements of the general boost converter (shown in FIG. 1) are indicated by the same reference numerals, and descriptions thereof are omitted. The prior art boost converter has the same construction as that of FIG. 1 except that it further comprises a snubber circuit. The snubber circuit comprises a second inductor 21 connected between the first connection point P1 and the first diode 14, a resistor 23 connected between the first connection point P1 and a second connection point P2, a second diode 24 connected in parallel to the resistor 23, and a condenser 22 connected between the second connection point P2 and the positive terminal (-) of each of the portions V.sub.i and V.sub.o.
Turning again to FIG. 1, if the switching device 13 is turned on while a current passes through the first diode 14, a very large reverse recovery current flows through the switching device 13 for a reverse recovery time. Then, due to the high large reverse recovery current, the switching device 13 inevitably becomes damaged and also energy loss of the boost converter is increased. Thus, the boost converter shown in FIG. 2 further comprises the inductor 21 capable of suppressing the reverse recovery current owing to the inductance 21 connected between the first inductance 12 and the first diode 14.
On the other hand, when the switching device 13 is turned off, the magnitude of a current flowing through the current suppressing inductor 21 can be reduced and thereby an excessive voltage is applied to both terminals of the switching device 13. Thus, to prevent occurrence of the excessive voltage at both terminals of the switching device 13, the boost converter further comprises the resistor 23 and second diode 24 which are connected in parallel between the points P1 and P2, and the condenser 22 which is connected between the second point P2 and the negative terminal (-) of the output portion V.sub.o, as shown in FIG. 2.
As described above, the prior art boost converter shown in FIG. 2 has the snubber circuit to absorb a stress energy such as an excessive voltage stress or an excessive current stress. For example, the current suppressing inductor 21 in the snubber circuit is capable of absorbing a stress energy occurring upon the switching device 13 being turned on, and the condenser 22 is capable of absorbing a stress energy occurring upon the switching device 13 being turned off. Also, if the switching device 13 is again turned on, the stress energy absorbed in the condenser 22 is discharged through the resistor 23.
However, since the absorbed stress energy in the condenser 22 is discharged as a thermal energy through the resistor 23, there are two problems in the prior art boost converter using a snubber circuit, one is allowing the total energy transfer efficiency thereof to be reduced, and the other is needing an additional circuit for preventing the discharging thermal energy from negatively affecting the other circuit elements in the boost converter.